Men are, on average, taller than women.
On a conventional tandem[1], stability (particularly when starting and stopping) is greatly improved by having the taller rider with the strongest arms on the front: Their job is to support the bike and the stoker until it starts bicycling. The stoker's job (in addition to being Always Right) is to provide a burst of power to get it up to speed as quickly as possible, while the pilot controls the wobble and sorts their own feet out.
But also, patriarchy.
[1] The small-wheeled Circe Helios is a notable exception, and is extremely forgiving of short pilots and heavy stokers. MTB geometries tend to be a bit less wobbly, too. Different rules apply with recumbents, trikes and other exotica, where leg length or ability to shift weight around when cornering may be a more important factor.